The present invention relates generally to a method and apparatus for introducing ions into analytical and other types of instruments. Specifically, the present invention provides an improved sample inlet tube to achieve improved efficiency in transferring ions from an ionization source at a relatively high pressure, such as a electrospray ionization source, into the interior of a device at a relatively low pressure, such as a mass spectrometer.
Many instruments utilize an ion stream produced at a given pressure which are introduced into an apparatus or instrument having an interior maintained at a somewhat lower pressure. For example, many types of analytical equipment, particularly mass spectrometers, utilize ions produced at atmospheric pressure by a standard electrospray ionization source, or xe2x80x9cESI.xe2x80x9d An ESI is a small capillary into which a liquid analyte is injected at one end and wherein a stream of ions emerges from the opposite end. The ESI is typically maintained at some voltage and positioned proximate to a small aperture through a device, such as a mass spectrometer. The aperture serves as the entrance for ions into the device. The interior of the device, or vacuum chamber, is typically pumped to a pressure well below the pressure of ions leaving the ESI, thereby sucking some proportion of the ions through the aperture and into the vacuum chamber of the device at very high rates of speed. It is believed that the high rates of speed at which the ions travel, and the electrical forces surrounding the transfer of ions leaving the tip of the ESI and entering the device, typically create a highly turbulent flow in the ion stream flowing between the ESI and the entrance to the device. Unfortunately, this turbulent flow often results in many of the ions generated within the ESI failing to flow into the vacuum chamber of the device. In fact, it is believed that it is not uncommon for ESI sources to have less than one percent (1%) of the ions generated at the tip of the ESI be introduced into the vacuum chamber of the device. This inefficient transfer of ions into the device creates a variety of shortcomings which render it highly desirable to achieve much greater ion transfer efficiency. For example, increased efficiency in ion transfer allows a analytical instruments such as mass spectrometers to achieve much higher detection sensitivity. Those having skill in the art have long recognized the advantages of increased efficiency in ion transfer from standard ESI sources, and have long sought methods and techniques designed to increase this efficiency. Still, there exists a need for improved methods and apparatus which improve the efficiency in ion transfer.
Accordingly, it is an object of the present invention to provide a method and apparatus which provides greater efficiency in the transmission of gaseous ions from an ion source situated in a region of relatively high pressure, to the interior of a device maintained at a relatively low pressure.
It is a further object of the invention to provide a method and apparatus which provides greater efficiency in the transmission of gaseous ions from an ion source utilizing thermal ionization, ion beams, electron impact ionization, laser irradiation, ionspray, electrospray, thermospray, inductively coupled plasmas, microwave plasmas, glow discharges, arc/spark discharges, hollow cathode discharges, gases generated by evaporation of condensed substances, laser ablation of condensed substances and mixtures thereof to form ions in a region of relatively high pressure, which are then transferred to the interior of a device maintained at a relatively low pressure.
It is a further object of the invention to provide a method and apparatus which improves the efficiency of the transmission of gaseous ions from an ion source in a region of relatively high pressure, which are then transferred to the interior of a device such as a linear quadrupole mass spectrometer, an ion trap quadrupole mass spectrometer, an ion cyclotron resonance mass spectrometer, a time of flight mass spectrometer, or an electric and/or magnetic sector mass spectrometer, which is maintained at a relatively low pressure.
These and other objects of the invention are accomplished by providing an improved inlet tube for transmitting ions from an ion source maintained in a region of relatively high pressure, to a device having an interior maintained at a relatively low pressure. The improved inlet tube is positioned within an aperture through the device to allow the passage of ions from the ion source, through the improved inlet tube, and into the interior of the device. The inlet tube is designed with a larger end and a smaller end wherein the larger end has a larger interior diameter than the interior diameter of the smaller end. The inlet tube is positioned within the aperture such that the larger end is pointed towards the ion source, to receive ions therefrom, and the smaller end is directed towards the interior of the device, to deliver the ions thereto. Preferably, the ion source utilized in the operation of the present invention is a standard electrospray ionization source. However, as will be apparent to those having skill in the art, the practice of the present invention may also be practiced with any method useful for generating ions, including, but not limited to, thermal ionization, ion beams, electron impact ionization, laser irradiation, ionspray, electrospray, thernospray, inductively coupled plasmas, microwave plasmas, glow discharges, arc/spark discharges, hollow cathode discharges, gases generated by evaporation of condensed substances, laser ablation of condensed substances and mixtures thereof. Similarly, the present invention finds particular utility in conjunction with analytical devices such as mass spectrometers, including but not limited to magnetic-bottle time of flight photoelectron spectrometers, linear quadrupole mass spectrometers, ion trap quadrupole mass spectrometers, ion cyclotron resonance mass spectrometers, time of flight mass spectrometers, and electric and/or magnetic sector mass spectrometers. While the general nature and operation of the present invention is thus shown and described, a more in depth understanding of the invention may be acquired through a discussion of some preferred embodiments of the present invention. More particularly, a discussion of prototypes of two such preferred embodiments and experiments with these prototype devices provides an illustrative example of the efficacy of the present invention.